Figure 1 . --Distribution of the plankton at the time of the first survey 

 (July 9-August 12). 1 - southern boundary of the North Pacific Tem- 

 perate Region (the zone of Calanus tonsus); 2- northern boundary of 

 the Tropical Circumequatorial Region (tropical zone); 3 - boundary 

 of the subtropical fauna. The solid lines without marks indicate the 

 boundary of the transitional zone with Calanus pacificus; the broken 

 lines show the surface isotherms for July 9 to August 12, 1953. 



the 15°C. and 18°C. surface water isotherms 

 in July. These isotherms also outline very 

 well the northern frontal zone of the Kuroshio. 



South of this transitional zone lies a 

 narrow band of the subtropical fauna. In July 

 this fauna is found mainly in the western part 

 of the area of investigations, that is, approxi- 

 mately in line with the islands of Shikotan, 

 Iturup, and Kunashir, but in September it 

 spreads to the east, almost to the line of Cape 

 Lopatka in Kamchatka. 



The transition between the zone of Calanus 

 pacificus and the belt of subtropical fauna is 

 relatively abrupt: the differences in the com- 

 position, in the color of the plankton sediment, 

 and in its volume are clearly noticeable, even 

 between adjacent stations 40 miles apart. This 

 boundary is also revealed by the appearance of 

 such organisms as Physalia, and also by num- 

 erous salps, Pyrosoma , flying fish, blue 

 sharks, sunfish, and turtles. 



Beyond the narrow belt of subtropical fauna 

 begins the true tropical fauna, abounding in 

 various groups and numerous species. Among 

 the copepods we encounter here Undinula 

 darvinii, U, vulgaris, Nannocalanus minor, 

 Neocalanus robustior, Canthocalanus pauper , a 

 series of species of the genus Eucalanus (E. 

 crassus, E. subcrassus, E. nnonarchus, E. 

 attenuatus), Clausocalanus arcuicornis with its 

 three varieties, Calocalanus pavo , Corycaeus , 

 Copilia, Sapphirina, species of the genera 

 Centropages, Candacia, et cetera. There are 

 numerous siphonophores, salps, Lucifer, Ptero- 

 poda, and Carinaria and other genera of Gastro- 

 poda; one is struck by the abundance of Chaeto- 

 gnatha; and there are numerous radiolarians of 

 the Acanthometridae. Here there are more than 

 100 species of calanoids alone, while in the zone 

 of Calanus tonsus there were only 5 or 6. The 

 plankton biomass here is significantly less than 

 in the zone of abundance of Calanus tonsus. 



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